Adjustable shunt regulated and switching power supply for loop powered transmitter

Abstract

A loop powered process instrument comprises a control system measuring a process variable and developing a measurement signal representing the process variable. An output circuit, for connection to a remote power source using a two-wire process loop, controls current on the loop in accordance with the measurement signal. A power supply is connected to the output circuit and the control system. The power supply receives power from the two wire process loop and supplies power to the control system. The power supply comprises a voltage regulator receiving loop power and developing a regulated output voltage and an adjustable shunt regulator controlling voltage supplied to the voltage regulator.

The invention claimed is: 
     
       1. A loop powered process instrument comprising:
 a control system measuring a process variable and developing a measurement signal representing the process variable; 
 an output circuit, for connection to a remote power source using a two-wire process loop, for controlling current on the two-wire process loop in accordance with the measurement signal; and 
 a power supply, connected to the output circuit and the control system, for receiving power from the two-wire process loop and supplying power to the control system, wherein the power supply comprises a voltage regulator receiving loop power and developing a regulated output voltage, and an adjustable shunt regulator controlling voltage supplied to the voltage regulator. 
 
     
     
       2. The loop powered process instrument of  claim 1  wherein the adjustable shunt regulator is controlled by the control system. 
     
     
       3. The loop powered process instrument of  claim 1  wherein the adjustable shunt regulator is controlled to insure high input impedance to the instrument. 
     
     
       4. The loop powered process instrument of  claim 1  wherein the adjustable shunt regulator is controlled to increase voltage supplied to the voltage regulator as loop current decreases and is controlled to decrease voltage supplied to the voltage regulator as loop current increases. 
     
     
       5. The loop powered process instrument of  claim 1  wherein the control system adjusts a resistor ratio that feeds voltage supplied to the voltage regulator. 
     
     
       6. The loop powered process instrument of  claim 5  wherein the control system adjusts the resistor ratio in discrete steps. 
     
     
       7. The loop powered process instrument of  claim 6  wherein the control system selectively couples resistors of a resistor divider circuit to the adjustable shunt regulator. 
     
     
       8. The loop powered process instrument of  claim 5  wherein the control system adjusts the resistor ratio using a potentiometer. 
     
     
       9. The loop powered process instrument of  claim 1  wherein the control system adjusts the resistor ratio using a digital potentiometer. 
     
     
       10. The loop powered process instrument of  claim 1  wherein the control system comprises a programmed processor. 
     
     
       11. A two-wire transmitter comprising:
 a signal processing circuit measuring a process variable and developing a measurement signal representing the process variable; and 
 a control system including an output circuit, for connection to a remote power source using a two-wire process loop, for controlling current on the two-wire process loop in accordance with the measurement signal, and a power supply, connected to the output circuit and the control system, for receiving power from the two-wire process loop and supplying power to the control system, wherein the power supply comprises a switching regulator receiving loop power and developing a regulated output voltage, and an adjustable shunt regulator controlling voltage supplied to the switching regulator. 
 
     
     
       12. The two-wire transmitter of  claim 11  wherein the adjustable shunt regulator is controlled by the control system. 
     
     
       13. The two-wire transmitter of  claim 11  wherein the adjustable shunt regulator is controlled to insure high input impedance to the transmitter. 
     
     
       14. The two-wire transmitter of  claim 11  wherein the adjustable shunt regulator is controlled to increase voltage supplied to the switching regulator as loop current decreases and is controlled to decrease voltage supplied to the switching regulator as loop current increases. 
     
     
       15. The two-wire transmitter of  claim 11  wherein the control system adjusts a resistor ratio that feeds voltage supplied to the switching regulator. 
     
     
       16. The two-wire transmitter of  claim 15  wherein the control system adjusts the resistor ratio in discrete steps. 
     
     
       17. The two-wire transmitter of  claim 16  wherein the control system selectively couples resistors of a resistor divider circuit to the adjustable shunt regulator. 
     
     
       18. The two-wire transmitter of  claim 15  wherein the control system adjusts the resistor ratio using a potentiometer. 
     
     
       19. The two-wire transmitter of  claim 11  wherein the control system adjusts the resistor ratio using a digital potentiometer. 
     
     
       20. The two-wire transmitter of  claim 11  wherein the control system comprises a programmed processor.